This invention relates to elongated electrical heating elements and more particularly to a method of making such heating elements having uniform accurate resistance values.
Elongated electrical heating elements are commonly used in the injection molding field by integrally incorporating them in heated nozzles. One example is shown in the applicant's U.S. Pat. No. 5,282,735 which issued Feb. 1, 1994. It is well known to make elongated electrical heating elements by compacting a helical resistance wire in a powdered insulative material in an outer casing. As shown in U.S. Pat. No. 1,367,341 to Abbott which issued Feb. 1, 1921, it is also known to connect the inner ends of lead wires to opposite ends of a coiled resistance wire. Some high quality multi-cavity applications require the temperatures of all the different nozzles in the system to be the same. However, heating elements made by current methods have resistance deviations of up to plus or minus five to ten percent. This is not accurate enough to provide sufficiently uniform temperatures between the nozzles for many applications, with the result that separate temperature control stations must be provided for each nozzle in the system. This has the disadvantages to being more costly and subject to malfunctions.